In many applications of optics, it is required to transmit light-in the visible or infrared regions of the spectrum--through the atmosphere, or through space. This applies not only to optical communications where information is transferred from a transmitter to a receiver by means of optical signals, but also to various methods of optical alignment, measurement at distance, and imaging.
In many of these cases, the optical path includes mirror or prism reflectors which redirect the optical beam, often returning it to a receiver located in proximity to the transmitter. In general, precise alignment of the reflector is necessary, otherwise the reflected beam will miss the receiver.
In order to render reflector systems insensitive to slight misalignment, certain optical devices have been designed for directive reflection at predetermined angles of deviation. These are called constant-deviation devices. The most common of these is the retroreflector. A retroreflector is a device in which radiation is returned in the direction from which it came (180 degrees deviation), this property being maintained over a wide range of directions of incident radiation. The range of directions for which the device functions as a retroreflector may be limited to one of specific plane of incidence, as in a Porro prism. A retroreflector which is not limited to one specific plane of incidence usually comprises either three mirrors with mutually perpendicular surfaces, or a prism that forms the corner of a cube cut along a hypotenuse face. These configurations are known as corner reflectors. An additional example of a constant-deviation device, providing 90 degree deviation, is the pentaprism.
In a known reflector of this type, the mirrors are mutually adjustable. This enables reflection of incident radiation to directions other than the return direction, while preserving the characteristic of constant-deviation, i.e., that the angle between the reflected and incident beams (the deviation angle) remains constant over a wide range of directions of incident radiation.
Disadvantages of the above constant-deviation reflector include the following:
1. The adjustments of the mirrors are mutually dependent, and initial setup is therefore difficult, considerable calculation and subsequent manipulation being needed to guide the beam precisely to the required direction. Particularly, for each plane of incidence, each mirror must be positioned at a precise angular orientation relative to the others, and relative to the plane of incidence, in order to achieve stabilization. Each change requires a corresponding change in the mounting (and the housing) of each component to compensate for such a change.
2. The system is not very stable, and relatively minor environmental changes can affect the deviation angle, thereby rendering the system ineffective.
3. The mirrors are sensitive to contamination and other damage to the reflective surfaces.